There are a large number of processes in which two or more nonmiscible fluids are contacted in a countercurrent fashion to exchange mass, momentum, or heat between the various phases.
Transfer of mass between the phases often involves a technique, often called liquid or solvent extraction, which depends on the selective dissolution of one or more components of the solution into a suitable immiscible solvent. The process is particularly useful when separating mixtures which are difficult to separate using physical methods, e.g., distillation, because the vapor pressures or other physical parameters of the components are too close.
The petroleum refining industry is potentially the largest identifiable user of extraction processes. In refining, most lubricating oil feedstocks are extracted to remove the less-useful aromatic compounds from the desired paraffinic and naphthenic hydrocarbons. Liquid propane is also used to separate desirable oils from the less beneficial asphaltenes. Kerosene is treated using various caustic solutions to remove the bulk of the included sulfur compounds.
Other chemical industries also use this type of liquid extraction. Virtually all vegetable oils are separated into edible and inedible portions using liquid propane as the extractant. Most uranium is extracted from its liquid lixiviant with tributyl phosphate in the process to concentrate it into a condensed and useful form.
In addition to the attainment of mass transfer, direct countercurrent liquid (or gas) -liquid contact is used to promote heat transfer from one phase to another. For instance, variously in Kehat et al, British Chemical Engineering, Vol. 14, No. 6, pgs. 803-805; Letan et al, AIChE Journal, Vol. 14, No. 3, pp 398-405; and Letan et al, AIChE Journal, Vol. 16, No. 6, pp. 955-963, the design of direct heat exchangers, i.e., those having no solid surface between hot and cold fluids, is discussed at length.
The equipment used to bring about direct contact of one fluid with the other typically utilize internal components which bring about intimate contact between the liquids with a high degree of turbulence. The rate of mass or heat transfer between the fluids depends to a large extent upon the contact area between the fluids. The extractor disperses one of the fluids in the other to produce a large surface area and relative motion to produce turbulence. The equipment usually provides for the subsequent mechanical separation of the dispersion based upon the different densities of the fluids. In a solvent extractor, the two streams are conventionally known as the raffinate, i.e., the stream remaining after the extraction is performed, and the extract, i.e., the solvent containing the extracted constituents.
Specifically, several widely used fluid contacting devices are the mixer-settler, the packed-column, the perforated tray tower, rotary-disc contactor, York-Scheibel column, Treybal extractor, and pulsed column.
In the mixer settler, the two liquids flow through a mechanically stirred box. The intimate mixture then flows to a settler having low relative velocity so that the two phases will be allowed to coalesce and separate. Upper and lower phases may then be recovered.
The packed-tower equipment is made up of a generally hollow vertical tower having a support grid in the lower end, an upper liquid distributor and a lower fluid (light liquid or gas) distributor. The region above the support grid is filled with a packing material such as rings or saddles.
The perforated-tray tower is also a vertical vessel containing a number of trays having small perforation therein and, on each tray, a sort of upside down weir to collect and coalesce the light fluid as it travels from the next lower plate. The light fluid is alternately dispersed and coalesced as it traverses up the tower. For additional discussion of the above devices, See, e.g., R. E. Treybal, Mass Transfer Operations, 2nd ed., McGraw-Hill, 1968.
Rotary-disc contactors are discussed at length in U.S. Pat. No. 2,601,674. The contactor is a tower formed into compartments by horizontal annular baffles. Within each compartment is a rotating, centrally located, horizontal disc to provide agitation. The diameter of the disc is generally smaller than the diameter of the annular hole. The contactor is virtually always used in countercurrent flow.
The York-Scheibel column is of two similar designs, as found in U.S. Pat. Nos. 2,493,265 and 2,850,362. The earlier model utilized alternating chambers in a tower so that each open chamber containing a centrally located impeller is vertically adjacent to two chambers filled with open weave wire mesh packing. The later design includes a pair of stationary annular shroud baffles surrounding each moving impeller. Again, these are typically operated in countercurrent flow.
The Treybal extractor, found in U.S. Pat. No. 3,325,255, is described in Chemical Engineer's Handbood, 5th Ed., Perry and Chilton McGraw-Hill (1973), as an adaption of a mixer-settler cascade extractor to column form wherein the liquids are permitted to settle completely between extraction stages.
Finally, a pulsed column is an extractor in which a relatively short amplitude oscillation is applied to the liquid contents. The principle has been attributed to van Dijck, as shwon in U.S. Pat. No. 2,011,186. The tower itself is usually a packed column or the perforated tower collumn discussed above.
Another countercurrent fluid contacting device which has received only moderate attention is the so-called spray tower. This device is essentially an open vertical tower with a light fluid distributor located in the bottom of the tower. Possibly the most well known version of this tower design is the socalled Elgin tower shown in Blanding et al, Transactions of the AIChE, Vol. 38, pp. 305-338 (1942) and U.S. Pat. No. 2,364,892 to Elgin, issued Dec. 12, 1944. However, the obvious benefit to the design is that the liquid throughput rate can be far greater than similarly sized devices such as those discussed above. The lack of any internal structure in the portion of the device above the lower distributor allows the continuous phase to undergo significant axial mixing. Consequently, the heat or mass transfer rates generally are low because of the lack of true countercurrent temperature or concentration differences.
The benefits of using a spray tower are significant enough that considerable work has been done in attempting to enhance the efficiency of these devices. For instance, the articles mentioned above, i.e., Kehat et al in British Chemical Engineering and the two Letan et al in AIChE Journal, discuss at length the design of spray column heat exchangers and the mechanism of heat transfer therein. The relative volume, or "hold-up", of the dispersed phase within the column proper is an important feature of the column design and operation. The effectiveness of the column as a heat exchange device or as an extraction device depends, as noted above, on the maintenance of a stable droplet dispersion with high interfacial area for transfer of heat or mass between disperse and continuous phases.
Several discrete hold-up phenomena occur as the flow rate of the dispersed phase is increased:
First, a dispersed packing mode is observed characterized by a low dispersed phase hold-up of about 0-40 volume percent. This conditions is easily obtained and yields a swirling motion of droplets rising within the column.
Second, as the dispersed phase flow is increased and the coalesced zone of droplets is maintained at the top of the column, a dense packing of dispersed phase droplets builds up at the top and works its way down the column. This is a condition of high hold-up, with values ranging from 50 to 80 volume percent (depending, of course, upon the fluids utilized). The maximum theoretical hold-up obtained via close packing of undeformed uniform sized spheres is about 75 volume percent. However, the presence of smaller drops within the interstitial zones of larger droplets and the existence of distorted drops can yield holdups as high as 90 volume percent.
Finally, as the dispersed phase flow rate increases even further, flooding occurs. The concept of "flooding" is discussed in Unit Operations of Chemical Engineering, 3rd Ed., McCabe & Smith, McGraw-Hill (1976), p. 622 in the following manner: "If the flow rate of either the dispersed phase or the continuous phase is held constant and that of the other phase gradually increased, a point is reached where the dispersed phase coalesces, the holdup increases, and finally both phases leave together through the continuous-phase outlet." A selection of definitions for visual flooding is also offered in AIChE Jour., Vol. 13, No. 3, p. 448. The latter paper suggests that the McCabe & Smith definition is perhaps a more sensitive measure of the maximum effective capacity of a device.
Blanding et al, Elgin, both Letan et al articles and Kehat et al discuss the desirability of obtaining dense packing in the column. Similarly, Lackme, AIChE Symposium, Scr. 70, (130) 57 (1974), and Greskovich et al, AIChE Journal, Vol. 13, 1160 (1967) cite the improved results that are achievable when dense packing is utilized.
In sum, the bulk of the literature data associated with studies of low holdup packing operation in spray-type fluid contactors show overall poor contacting. Axial dispersion effectively increases rapidly as column diameter increases beyond 6 inches to the industrially significant range of 36-40 inches. Efficient heat transfer, at least, cannot be achieved if low holdup column operation is permitted.
Thus, in the operation of a spray column as a direct heat exchanger, the dense packing mode is essential to successful column utilization. In the dense packing mode, wake shedding phenomena are damped out and gross backflow of the continuous phase is reduced substantially. However, even with this type of operation, there are problems. Both Letan et al and Kehat et al describe the potential inefficiencies of high hold-up or dense bed packing. In this case, they were studying a system where the disperse phase was less dense than the continuous phase. Narrow, irregular and asymmetric swarms of drops can be noted adjacent to the column walls, moving with higher velocity than the drops in the central core. This contributes to a characteristic, but oddly skewed, residence time distribution for the total array of drops within a column. Bypassing of asymmetric channels of the continuous phase down the column walls can also be observed. The noted articles by Letan and Kehat modeled the dense packed column flow in three sectors: an outer (annular) channel of dispersed phase drops moves up the column in plug flow, a thin downward flowing annular layer of continuous phase fluid is located between the outer layer of rising dispersed phase drops and an inner central core of rising dispersed phase drops. In this central core, the rising dispersed phase flows with a radial velocity profile.
Clearly, even in the studies designed to optimize the operation of a device such as this, the studies have not produced a broad and useful general means of obtaining operation without substantial backmixing. Clearly, nowhere is there shown the stabilization of such a column by introducing into the continuous or dispersed phase (as appropriate) a magnetizable component which does not change substantially the physical parameters (save magnetic and density parameter) of the innoculated liquid, and imposing on at least a portion of the column containing the fluids, a magnetic field of strength sufficient to prevent substantial backmixing.